When fossil fuels are burned oxides of nitrogen (NO and NO.sub.2) result from thermal reaction of atmospheric nitrogen and atmospheric oxygen (thermal NO.sub.x), from reaction of nitrogen from the fuel with atmospheric oxygen (fuel NO.sub.x) and by conversion of compounds (radicals) that contain nitrogen with combustion air (prompt NO.sub.x).
The contents of oxides of nitrogen vary according to the fuel that is involved and according to combustion conditions. Once liberated to the atmosphere, the nitrogen oxides are subject to further chemical conversion, particularly photochemical conversion, which can subsequently lead to the formation of organic peroxide compounds with the hydrocarbons that are also present, and which in turn are responsible for extensive plant damage.
Together with the oxides of sulfur (SO.sub.2 and So.sub.3) that result from the sulfur compound in the fuels, as well as the acids that result from these, the oxides of nitrogen and their secondary products that are formed in the atmosphere are regarded as the principal cause of air pollution and the damage that results from such pollution.
At present there is a paucity of knowledge about the precise causal mechanisms of many of the types of such damage.
Legal regulations as well as guidlines and instructions have been enacted in various industrial nations with the aim of reducing emissions of the oxides of sulfur and nitrogen.
Catalytic process for the removal of oxides of nitrogen are known; in these processes NO and NO.sub.2 are reduced to simple nitrogen by reducing agents. In the combustion plants of power generating stations, for example, ammonia is used predominantly as the deoxidizing agent for oxides of nitrogen that are then converted to elementary nitrogen and water over various catalysts such as transition metal oxides, zeolites, activated charcoal, or activated coke.
Removal of sulfur oxides from the smoke gases is in many instances effected by contact with washing solutions that contain calcium ions that remove the oxides of sulfur, oxidized to sulfates, in the form of gypsum.